Spin wave mediated unidirectional Vortex Core Reversal by Two Orthogonal Monopolar Field Pulses: The Essential Role of Three-dimensional Magnetization Dynamics

TL;DR

This study demonstrates that three-dimensional magnetization dynamics are crucial for understanding vortex core reversal in nanodisks, showing how orthogonal monopolar pulses induce unidirectional switching through resonant spin wave amplification.

Contribution

The paper provides experimental and simulation evidence that three-dimensional effects are essential for vortex core reversal, highlighting the role of spin wave resonance and pulse orientation.

Findings

Unidirectional vortex core switching depends on pulse orientation.

Three-dimensional simulations match experimental results, unlike 2D models.

Resonant azimuthal spin waves modulate switching thresholds.

Abstract

Scanning transmission x-ray microscopy is employed to investigate experimentally the reversal of the magnetic vortex core polarity in cylindrical Ni81Fe19 nanodisks triggered by two orthogonal monopolar magnetic field pulses with peak amplitude $B_0$, pulse length ${\tau}$=60 ps and delay time ${\Delta}$t in the range from -400 ps to +400 ps between the two pulses. The two pulses are oriented in-plane in the x- and y-direction. We have experimentally studied vortex core reversal as function of $B_0$ and ${\Delta}$t. The resulting phase diagram shows large regions of unidirectional vortex core switching where the switching threshold is modulated due to resonant amplification of azimuthal spin waves. The switching behavior changes dramatically depending on whether the first pulse is applied in the x- or the y-direction. This asymmetry can be reproduced by three-dimensional micromagnetic simulations but not by two-dimensional simulations. This behavior demonstrates that in contrast to previous experiments on vortex core reversal the three-dimensionality in the dynamics is essential here.